Polyurethane foams have found extensive use in a multitude of industrial and consumer applications. This popularity is due to polyurethane's wide ranging mechanical properties and the excellent cushioning performance of the foamed product and its ability for the foam to be relatively easily manufactured. Furniture and mattresses, for example, rely on the durability and cushioning performance of polyurethane foams to provide comfort and support over years of use. Automobiles also, contain numerous polyurethane foam components, such as seats, trim and other interior parts. Polyurethane foams have traditionally been categorized as being flexible (or semi-rigid) or rigid foams; with flexible foams generally being softer, less dense, more pliable and more subject to structural rebound subsequent loading than are rigid foams. Flexible polyurethane foams have been further characterized as being conventional or high resilience (also described as high support); wherein the high resilience foams are produced from polyols having a high primary hydroxyl content, typically greater than 50%, and generally require crushing to fully open the foam and achieve good air flow. Conventional foams typically are produced by a free-rise slabstock process from polyols with low primary hydroxyl content, usually less than 10%, and do not require crushing to fully open the foam.
The production of polyurethane foams is well known to those skilled in the art. Polyurethanes are formed from the reaction of NCO groups with hydroxyl groups. The most common method of polyurethane production is via the reaction of a polyol and an isocyanate which forms the backbone urethane group. Cross linking agents, blowing agents, catalysts and other additives may also be included in the polyurethane formulation as needed.
Polyols used in the production of polyurethanes are typically petrochemical in origin, being generally derived from propylene oxide, ethylene oxide and various starters such as ethylene glycol, propylene glycol, glycerin, sucrose and sorbitol. Polyester polyols and polyether polyols are the most common polyols used in polyurethane production. For semi-rigid foams, polyester or polyether polyols with molecular weights of from about 300 to 2,000 are generally used, whereas for flexible, foams longer chain polyols with molecular weights of from about 1,000 to 10,000 are typically used. Polyester and polyether polyols can be selected to allow the engineering of a particular polyurethane elastomer or foam having desired final toughness, durability, density, flexibility, compression set ratios and modulus and hardness qualities. Generally, higher molecular weight polyols and lower functionality polyols tend to produce more flexible foams than do lower molecular weight polyols and higher functionality polyols.
Petroleum-derived components such as polyester and polyether polyols pose several disadvantages. Use of such polyester or polyether polyols contributes to the depletion of oil, which is a non-renewable resource. Also, the production of a polyol requires the investment of a great deal of energy because the oil to make the polyol must be drilled, extracted and transported to a refinery where it is refined and processed to yield the finished polyol. As the consuming public becomes increasingly aware of the environmental impact of this production chain, consumer demand for “greener” products will continue to grow. To help reduce the depletion of oil whilst satisfying this increasing consumer demand, it would be advantageous to partially or wholly replace petroleum-derived polyester or polyether polyols used in the production of polyurethane elastomers and foams with more versatile, renewable and more environmentally responsible components.
A number of companies have announced goals of a certain percentage of their products being based on renewable resources and preferences for products based on renewable resources have begun to appear in some government regulations. These factors combined with the ever escalating costs of petroleum-based products have given added impetus to the efforts to develop foam products based on various oils derived from plants.
Unfortunately, the use of the petroleum-based products is a highly developed industry and years of optimization have created products tailored to meet strict industry requirements. Thus, the attempted substitution of products based on renewable resources has been constrained by several factors including the difficulty of developing “drop in” type products which can be added without significantly affecting the processing characteristics and without substantial loss of product quality. For example, although castor oil-based polyurethanes have been known for decades, their use has generally been limited to a few applications such as hydrophobic coatings and certain sealants where the typical polyurethane properties are not required. There is a continuing need to develop polyethers based on these natural products which can meet industry requirements for foam quality and processability
Although the patent and technical literature contains many references related to the use of either castor oil or castor polyols (See J. H. Saunders and K. C. Frisch, Polyurethanes Chemistry and Technology II. Technology Part II (High Polymers Vol. XVI), Interscience Publishers, 1964, pages 32-37, See also references listed in WO 2004/020497), a large fraction of this art teaches the use of prepolymers to obtain a useful foam article. Although prepolymer technology is still used in some applications such as many types of coatings, the majority of manufacturers in the flexible foam industry now employ one-shot processes in which castor oil finds very little utility.
Another drawback to the use of polyols based on castor oil is that since the 1950's, these polyols have been produced by alkoxylating with potassium hydroxide catalysis. Despite the fact that KOH is a very good catalyst for the production of polyethers from propylene oxide and ethylene oxide using starters such a glycerin, trimethylolpropane and sorbitol, extensive side reactions occur with natural products containing an ester function. As those skilled in the art are aware, potassium hydroxide is a catalyst both for the alkoxylation and transesterification reactions. Thus, potassium hydroxide catalyzes hydrolysis and competitive transesterification reactions during the alkoxylation reaction creating a wide range of esters as the hydroxyl end groups are continually exchanged at the ester function thereby creating broad molecular weight distributions. These molecular weight distribution products can have deleterious effects on foams made from base-catalyzed polyols.
In the late 1990's, the polyol production industry embarked on a major change as double metal cyanide (DMC) catalysts started to displace potassium hydroxide as the catalyst of choice for the production of polyols used to make slab polyurethane. DMC catalysts do not appreciably catalyze the transesterfication reaction and thus for the first time, polyols based on natural product esters could be produced without the inherent transesterification obtained with potassium hydroxide.
Asahi Glass (Kokai H5-163342) reported the production of EO/PO based polyethers using castor oil as a starter. The polydispersities of the resultant polyether products confirmed that a substantial change had occurred given that the obtained polydispersities were in the range of 1.10 to 1.13; whereas, the corresponding potassium hydroxide-catalyzed polyols had polydispersities in the range of 1.7 to 1.8. For the first time, an economical method had been developed for the production of polyethers based on renewable resource esters. Unfortunately, Asahi only reported the production of the polyethers and was silent on the suitability of those products in flexible polyurethane foams.
U.S. Published Patent Application 2006/0167125 discloses a method for producing low-emission polyurethane soft foams with a polyether alcohol prepared by addition of alkylene oxides onto compounds derived from renewable raw materials using a DMC catalyst. The polyether polyols suitable for the production of conventional slabstock polyurethane foams must have a high content of secondary hydroxyl groups and an ethylene oxide content in the polyether chain of no more than 30%. The polyether polyols preferred for the production of molded flexible polyurethane foams have a primary hydroxyl content of greater than 50% and particularly those with an ethylene oxide block at the end of the chain or be based solely on ethylene oxide. The foams produced in accordance with the method disclosed in U.S. 2006/0167125 are taught to have reduced crack formation and reduced compressive sets. U.S. 2006/0167125 does not, however, teach that free-rise, slabstock polyurethane foams having good processing characteristics could be produced with polyether polyols having a primary hydroxyl group content of greater than 10%. In particular, the teachings of this patent would not suggest that near “drop in” conventional slabstock foam processing and properties could be achieved at primary OH levels greater than 10% and ethylene oxide contents above 30%.
The belief that polyols with low primary OH (<10%) content and low ethylene oxide content (<20%) are preferred for conventional slabstock foam production is also supported in that essentially all major commercial petroleum based conventional slabstock polyols fall within these ranges.
Therefore, a need continues to exist in the art for flexible conventional polyurethane slabstock foams made with environmentally-friendly, renewable components having primary hydroxyl group contents of greater than 10% where such components will provide foam properties and near “drop-in” processing as replacements for petrochemical based polyols.